Frequency reuse is one of methods for increasing channels per unit area in a cellular system. The intensity of an electric wave becomes weak gradually as the distance becomes long. Accordingly, since there is little interference between electric waves in a place away from a given distance, a single frequency channel can be used therein. In this way, one frequency can be used simultaneously in various zones. This could increase subscriber capacity. Efficient use of the frequency will be referred to as frequency reuse. A unit for dividing zones from one another will be referred to as a cell (mobile communication cell), and frequency channel conversion between cells for maintaining communication will be referred to as hand-off. In an analog cellular mobile communication system, the frequency reuse technique is necessarily required. Namely, although the frequency reuse technique is used in an FDMA or a TDMA, since a total of cells use the same frequency in a CDMA, frequency reuse is not required. Accordingly, in this case, a frequency reuse coefficient or a frequency reuse rate is 1. The frequency reuse rate means one of parameters representing how frequency efficiency is in a cellular system, etc. The frequency reuse rate is a value obtained by dividing the number of cells (sectors) which simultaneously use the same frequency channel in a multi-cell structure by a total number of cells (sectors) of the multi-cell structure.
A frequency reuse rate of a 1 G system (for example, AMPS) is smaller than 1. For example, for 7-cell frequency reuse, a frequency reuse rate is 1/7. A frequency reuse rate of a 2 G system (for example, CDMA and TDMA) has been more improved than that of the 1 G system. For example, in a GSM where FDMA and TDMA are used in combination, a frequency reuse rate reaches ¼ to ⅓. In case of a 2 G CDMA system and a 3 G WCDMA system, since a frequency reuse rate can reach 1, spectral efficiency is increased and the network arrangement cost is reduced.
When all sectors within one cell and all cells within one network use the same frequency channel, a frequency reuse rate of 1 can be obtained. However, obtaining a frequency reuse of 1 in a cellular network means that edge users of cells have signal receiving throughput reduced by interference from neighboring cells.
Since a center region of the cell is close to a base station, it is safe from co-channel interference from neighboring cells. Accordingly, inner users in the center of the cell can use all possible sub-channels. However, users in the edge of the cell can use only a part of all possible sub-channels. In the edge of neighboring cells, a frequency is allocated in such a manner that respective cells use different sub-channels. This is called fractional frequency reuse (FFR).
In an OFDMA, since a channel is divided into sub-channels, a signal is transmitted on the sub-channels, and all channels are not used unlike 3 G (CDMA2000 or WCDMA). The FER scheme uses these features of the OFDMA. The FER maximizes frequency efficiency for users located in the center of the cell and improves throughput and signal intensity for users located in the edge of the cell (cell boundary).
As compared with a network where the same transmission power is used for all subchannels, sub-channels of high power and sub-channels of low power are used in combination to increase total coverage of the network. The base station can transmit data to short-distance subscribers using some sub-channels having low transmission power. The other sub-channels can be used with higher transmission power, and can be used for long-distance subscribers and short-distance subscribers. In order to minimize interference for subscribers of neighboring base stations, two neighboring cells can be configured in such a manner that high power is not equally allocated to same sub-channels. In this way, cell edge mobile stations can be used with low power by neighboring sectors, or can be scheduled by high power tone which is not used by neighboring sectors. According to this approach, all base stations use frequency bandwidths using different power levels on different sub-channels. Some tones are used by all sectors, and thus have a reuse rate of 1. On the other hand, since the other tones are used by only ⅓ of the sectors, the tones have a reuse rate of ⅓.
Examples of the FER scheme include a hard FER scheme and a soft FER scheme. In the soft FER scheme, some tones are used with low power. By contrast, in the hard FER scheme, some tones are not used at all.
Meanwhile, in a network according to the related art, communication has been directly performed between the base station and a receiver in all cases of downlink and uplink. On the other hand, a relay is additionally provided in a relay network. The relay reduces dependence on the base station. If the relay is used with reducing the number of base stations, the network cost can be reduced.
In a cooperative MIMO relaying network, a virtual antenna array is formed, and the relay is used to retransmit a signal transmitted from the base station in case of a downlink.
Cooperative MIMO has been developed from the cooperative relay technique. In cooperative relay, the relay should necessarily have the capability of decoding a received signal. The relay determines a transmission type in cooperation with the base station or other relays to provide the most excellent channel status to subscribers after decoding the received signal. If this relay is used, it is possible to increase cell capacity as well as cell coverage corresponding to the advantage of the relay.
Furthermore, in MIMO spatial multiplexing (SM), better throughput is obtained if correlation between values within a channel characteristic matrix formed among a plurality of antennas is low. Namely, if transmitting antennas are far away from one another and receiving antennas are also far away from one another, a good channel characteristic matrix is formed. According to this principle, if transmitting antennas are located in different places (for example, base station and relay), a good channel characteristic matrix is formed, whereby characteristics of MIMO SM can be maximized. In the MIMO system, if the antennas are far away from one another, it is preferable to improve diversity gain using the plurality of antennas as well as SM characteristics. In a multi-cell environment, neighboring cells of edge users located in the edge of the cell can perform cooperative MIMO.
In the hard FER scheme, among all possible FER bands, a FER band to which is not allocated to a cell is not used by a mobile station within the cell, whereby resource efficiency is deteriorated. Also, in the soft FER scheme, some regions are allocated to a user within a cell using low power to improve resource efficiency. In this case, an efficient method is required to perform cooperative MIMO having more excellent throughput.